Method for Producing a Multilayer Coating and Device for Carrying Out Said Method

ABSTRACT

A method for reducing the optical loss of the multilayer coating below a predetermined value in a zone by producing coating on a displaceable substrate in a vacuum chamber with the aid of a residual gas using a sputtering device. Reactive depositing a coating on the substrate by adding a reactive component with a predetermined stoichiometric deficit in a zone of the sputtering device. Displacing the substrate with the deposited coating into the vicinity of a plasma source, which is located in the vacuum chamber at a predetermined distance from the sputtering device. The plasma action of the plasma source modifying the structure and/or stoichiometry of the coating, preferably by adding a predetermined quantity of the reactive component to reduce the optical loss of the coating.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/537,224, filed on Nov. 16, 2005 (now U.S. Pat. No. 8,956,511, issuedon Feb. 17, 2015), which is a 35 U.S.C. §371 national stage entry ofPCT/EP2003/013649 filed on Dec. 3, 2003, which claims priority to GermanPatent Application No. DE 102 56 877.4 filed on Dec. 4, 2002 and GermanPatent Application No. DE 103 47 521.4 filed on Oct. 13, 2003. Thecontents of each of the above applications are hereby incorporatedherein in their entireties by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method for producing thin coatings and to anapparatus for the practice of the method.

Many different methods are known in the field of physical and chemicaldepositing technologies for the production of thin coatings. Differentmethods are used according to the desired properties of the coating tobe deposited and on the material systems chosen.

The method of cathode sputtering is advantageous, especially formaterials of high melting point, in which a plasma is ignited in avacuum in a range that is clearly above any typical residual gaspressure for vapor depositing processes, by using an electrical fieldfrom which ions are accelerated against a target that is at a highelectrical cathode potential, and these ions knock atoms out of thetarget which then deposit themselves on the walls of the vacuum chamberand on a substrate usually at ground potential or at a low bias voltage,which is situated at a distance from the target. Heating of the materialsource is not necessary, and instead the target is cooled during theprocess. The result is a residual gas pressure usually mostly of aninert gas such as, say, argon which has no unwanted influence on thecoating that forms on the substrate. For the deposition of compoundssuch as nitrides, carbides or oxides or the like, appropriate reactivegases can be admixed additionally to the sputtering gas.

The substrate is usually arranged outside of the plasma zone in order toprevent any damage to the freshly growing coating by radiation from theplasma or by residual sputtering effects. The average free length oftravel of the ions must be great enough so that they can reach thetarget with sufficient kinetic energy, i.e., with minimum interferencedue to further collision processes in the residual gas, which sets amaximum limit on the possible residual gas pressure. On the other hand,the pressure must be high enough to be able to ignite a stable plasma.With magnetic field-supported cathode sputtering it is possible toproduce an elevated electron density on the target, resulting in a highplasma density at the target and therefore a greatly elevated sputteringrate.

By the addition of reactive components, especially oxygen, to the inertgas, oxides can also be produced. Such a reactive sputtering process isdisclosed, for example, in WO 01/73151 A1, where the oxygen partialpressure during the sputtering of the oxide must be controlled by meansof a lambda probe, so that a stoichiometric oxide can form in thegrowing coating. Of course, the target also reacts with the reactivegas, so that competing processes, namely ablation on the one hand, andthe formation of oxide on the target surface to inhibit the ablation.This in turn has repercussions on the electrical potential in thecoating chamber, the formation of plasma, and the like. Likewise, thecoatings of the sputtered material also form, getter surfaces which bindoxygen, for example, as a reactive component and thus lead to a mutual,hard to predict interdependence of a variety of process parameters. Heretoo the relationship among the coating parameters is very complex. Oftenthere is then a mutual influence when just one coating parameter isvaried. Depending on the coating material to be deposited, it istherefore necessary to attune the coating processes and the coatingparameters to one another. This is all the more true the more complex alayer system to be deposited is, say, in the case of the deposition ofmultiple layers having special functional properties, especially opticalflinction coatings. The problems mentioned are especially pronounced inthe so-called reactive DC magnetron sputtering of metallic compounds, inwhich the requirement of a reacting compound on the substrate surface inthe case of a metallic target surface can be achieved only with greatexpense. For the production of insulating coatings, such as, e.g., SiO₂,Al₂O₃ and the like, methods have therefore already been developed inwhich, by means of two pairs of magnetron sputtering cathodes suppliedby an alternating current source, two targets are used in alternation.The polarities of the target potentials usually vary in the kHz range,i.e., each cathode is alternately cathode and anode. This leads to adefinite charge transport between cathode and anode without thehampering effect of an oxide coating on the target surfaces, in contrastto the disturbing effect of the so-called “disappearing anode” in thecase of reactive DC magnetron sputtering.

Efficient operation, however, requires operating in the so-calledtransition area since otherwise the formation of oxide on the targetsurface is faster than the ablation rate.

EP 0 795 623 A1 discloses an apparatus for the application of thintitanium oxide coatings by reactive cathode sputtering. Accordingly thepower supply to the cathode is regulated by the signal from a ÿ probesensor which compares the content of oxygen in the vacuum chamber with areference gas. The method is especially suited to the long stabledeposition of oxides, which are to be made as uniform as possible, withan unvarying composition.

DE 42 36 264 C1 discloses a plasma-supported electron beam vapordeposition in which an oxide is vaporized at a very high rate by anelectron beam vaporizer and deposited on a substrate. During thevaporization, however, the oxide dissociates so that the oxygen is lostand is no longer available for oxidation in the growing coating. Betweensubstrate and vaporization source there is therefore a plasma spacecontaining an oxygen plasma, in which the vapor is excited on the way tothe substrate, so that a stoichiometric oxide can deposit itself on thesubstrate. Depending on the material system, the deposition of astoichiometric oxide is successful since either the partial pressure ofthe reactive gas or the plasma parameters are regulated during thecoating process.

The relationships are very complex and can hardly be transferred fromone material system to another. Variation of individual processparameters produces different results in different material systems.Deposition parameters optimized for aluminum oxide, for example, do notyield optimum results, in the case of silicon oxide, for example.Moreover, different vaporization parameters which can not be ascertainedseparately appear also within one and the same material system, whichlead to undesired alterations of the properties of the depositedcoatings and make the repeatability of a started coating processadditionally difficult.

In EP 0 1516 436 E1 a magnetron sputtering apparatus is disclosed forthe reactive depositing of a material onto a substrate with a magnetronsputtering apparatus and a secondary plasma apparatus. The sputteringsystem and the secondary plasma apparatus have each sputtering andactivation zones which are atmospherically and physically adjacent. Bybringing together the sputtering and activation zones, the plasmas ofboth zones are mixed to form a single, continuous plasma.

In EP 0 716 180 B1 a coating apparatus is disclosed, with a depositionsystem and an apparatus for producing a plasma. The deposition andplasma apparatus can be operated selectively, so that a compositionlayer is formed which has at least several layers. The composition ofeach layer can be chosen from at least one of the following substrates:a first metal, a second metal, an oxide of the first metal, an oxide ofthe second metal, mixtures of the first and second metal and oxides ofmixtures of the first and second metal.

SUMMARY OF THE INVENTION

The problem of the invention is to provide a method for the productionof thin coatings, by which the composition of the coating can becontrolled and influenced, and create an apparatus for the practice ofthe method.

It is furthermore the problem of the invention to create a method and anapparatus for producing an optically low-loss multilayer coating,especially by the above-described method of the invention.

The problem is solved by the features of the independent claims.

In contrast to the oxidation of metal coatings or semiconductorcoatings, the targeted deposition of a hypostoichiometric coating in thevicinity of a sputtering apparatus permits an increase of the coatingrate, since the subsequent plasma treatment can oxidize thicker coatingsto the stoichiometric oxide in less time. Furthermore, the reactivedeposition according to the invention permits the reactive deposition ofa coating according to the invention with a predetermined thickness withan optical loss lower than a given minimum, and subsequent plasmatreatment permits a relative fast preparation of coatings with lowoptical losses. In comparison to known reactive sputtering processes,the sputtering process is less ended by troubles such as flashovers orcathode arcing, while at the same time coatings of high quality areformed.

According to a further aspect of the invention, in the case of a processfor producing a multilayer coating with at least one reactively operatedcoating apparatus and at least one reaction apparatus in a vacuumchamber on at least one substrate moving relative to the said apparatus,the deposit of a second coating with at least one reactive component isperformed. By means of the reaction apparatus a change of the structureand/or the stoichiometry of the coatings takes place. To lessen anyoptical loss of the multilayer coating below a given value, provision ismade according to the invention to construct an interface in an area ofthe second layer adjoining the first layer by means of the coatingapparatus, having a thickness d₁ and a value of a deficit of thereactive component DEF that is less than a value DEF₁. This methodpermits a comprehensive control of the changing of the structure and/orstoichiometry of the coatings and to produce preferably multilayercoatings with low optical losses and a low optical reflection and hightransmissions.

The method of the invention for the production of multilayers withminimal optical losses sets out from the knowledge that the depositedcoatings should have insofar as possible a complete stoichiometrybetween a first constituent and a reactive constituent. According to theinvention the reactive sputtering process is conducted in a mode ofcontrolled substoichiometry and in a second step the lacking content ofthe reactive constituent is made available by the action of theadditional plasma source. For example, the sequence for producingstoichiometric SiO₂ coatings is as follows: in a first step a sputtercoating is performed using a metallic silicon target, a reactive gasflow of oxygen being used and leading to a substoichiometric compound ofa coating of, for example, SO_(1.6). The corresponding value of thedeficit of the reactive component DE is then 0.4. In a second step, aplasma activation is performed with oxygen as reactive gas, which leadsto a fully stoichiometric SiO₂ coating.

The deposit of such coatings is determined by parameters which arematerial-related. In this case it is always possible, according to theinvention, to assure an optimum compromise between a high sputteringrate and a maximum achievable stoichiometry in a first step, combinedwith a maximum effective following treatment by the plasma source in thesecond step.

According to the invention, in the production of coatings, especiallymultilayer coatings with a high-refraction layer and an adjoininglow-refraction layer, in order to avoid optical losses. These measuresare preferentially indicated, since due to the extremely high reactivityof silicon, a substoichiometric compounds, such as SiO_(1.6) forexample, which is sputtered onto a fully stoichiometric Nb₂O₅ layer,oxygen is removed from the Nb₂O₅ coating before the SiO_(1.6) layer hasbeen changed by the plasma source to a fully stoichiometric state. Thisleads to an impairment of the optical properties of at least some layersof the high-refraction material and thus to a degradation of themultilayer coating, especially in proportion to the number of interfacesinvolved. To prevent the described effect, in for example an area of theinterface directly adjacent the high-refraction material, alow-refraction layer of a certain thickness is provided according to theinvention, which is in a largely or completely stoichiometric state. Forexample, this layer can typically have a thickness of 3.6 nm in the caseof SiO₂. This area of the interface acts as a barrier for the protectionof the high-refraction layer underneath it. As soon as a criticalthickness of the said area has been reached the parameters of thesputtering process can be changed in the direction of the deposit ofcoatings of a high degree of substoichiometry or a higher value of thedeficit DEF of the reactive component. Accordingly, an SiO₂ coating madeby the method of the invention has an internal structure, wherein afirst part has a slight oxygen deficit and a second part a higher levelof the deficit.

Preferentially, the production of multilayer coatings with layers ofhigh-refraction and low-refraction material alternating one on theother. Preferentially, Nb₂O₅, Ta₂O₅, ZrO₂ or Al₂O₃ and, aslow-refraction material, SiO₃, are provided.

With the method of the invention, thin coatings are successfullydeposited with high precision and excellent quality. In an especiallypreferred embodiment the creation of oxide, carbide and nitride coatingsof high optical quality is successfully accomplished.

Additional configurations and advantages of the invention are to befound in the further claims and, independently of their summation in theclaims, in the description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described with the aid of drawings, wherein thefigures show schematically:

FIG. 1 A representation of a preferred arrangement of substrate, targetand plasma source for the oxidation of a coating.

FIGS. 2 to 5 Characteristic curves in a reactive cathode sputtering.

FIG. 6 An example of the optical transmission of a coating as a functionof the wavelength, with and without the substrate speed, as parametersof the set of curves.

FIG. 7 An example of the optical transmission of a coating as a functionof wavelength with the substrate speed as a parameter of the set ofcurves.

FIG. 8 Shows examples of optical losses of individual layers withdifferent coating thicknesses.

FIG. 9 The structure of a multilayer coating with interface.

FIG. 10 The influence of the number of interfaces in a multilayercoating on transmission and reflection.

FIG. 11 The effect of minimizing the optical losses for a multilayercoating according to the method of the invention.

FIG. 12 Optical losses for single and multiple layer coatings ofmaterial of high and low refraction and for different coating thickness;the curves marked A and B correspond to multilayers with 77 and 21interfaces without interface optimization; the curves marked C, D, E, F,correspond to single coatings; a multilayer coating and 77 interfacesand interface optimization is identified by XY.

FIG. 13 Transmission and reflection for a multilayer coating with lowoptical losses, for wide band filters, for example.

FIG. 14 Optical losses for a multilayer coating for various thicknessesof an interface. A corresponds to a thickness of 2.7 nm, B to athickness of 3.6 nm.

FIG. 15 Characteristic curves in the case of a reactive cathodesputtering of silicon at powers of 1 KW and 1.5 KW.

FIG. 16 Curves of various process parameters in a reactive cathodesputtering of silicon.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With the method of the invention, coatings with low optical losses canbe made by reactive sputtering without breaking the vacuum, whichcontain reaction components such as oxygen, carbon or nitrogen. Aproduction of oxides is described hereinafter; the method is, however,also suitable for carbides or nitrides or mixtures such as oxynitridesor carbonitrides or the like, while also 2 or more reactive gases can beused simultaneously as reactive components.

FIG. 1 shows a schematic drawing of a preferred system for depositing acoating according to one embodiment of the invention. An oxide coatingis deposited with a residual gas on a substrate 2 in a vacuum chamber10. The vacuum chamber 10 is divided into several areas, A, B and C.Preferably each area A, B and C has its own gas delivery, not shown, aswell as its own pump supply. Also, more than three such areas can beprovided. Areas A, B and C are preferably divided off from one anotherby diaphragms which are connected to one another only through slits.Thus, a separation by vacuum is achieved, of process stations to bedescribed further on, namely sputtering systems or a plasma source, intoplasma areas A, B and C to be described further on. Preferably theprocess stations are not connected plasma-wise. The pressures,preferably the partial pressures of gases in the residual gas (sputtergas) in the apparatus can be adjusted largely independently of oneanother. Preferably, an inert gas such as argon Ar and a reactive gas,preferably oxygen O₂ are contained in the residual gas.

In the area A, a first cathode sputtering source is provided as thesputtering system 3, preferably a magnetron source system with twomagnetron systems side by side, also known as a “Twinmag.” The powersupply can be a DC, DC pulsed or middle frequency (MF) or high frequency(HF) or a combination DC-HF power supply. Typical voltage ranges of thesputtering cathode are 400 V to 500 V. Preferably an MF source with 40kHz is used.

In area A a sputtering material is sputtered from a target by reactivesputtering, a combination of sputtering material and oxygen is depositedat a sputtering rate depending on the working point on walls of thevacuum chamber 1 and on the substrates 2. Preferred sputtering materialsare metals and metal alloys, such as Al, Nb, Hf, Ta, Ti, Zr, TiNb, andsemiconductors such as Si.

In area B a plasma source 5 is arranged which produces a plasma whichcontains excited ions and radicals of the reactive component of theresidual gas. The reactive particles act upon the deposited coating andfurther oxidize it. The plasma source 5 can be, for example a DC, HF, MFor DC pulse or DC+HF microwave plasma source apparatus, especially aHall End plasma source, a hot cathode DC plasma source, a high-frequencyplasma source, a medium-frequency or a pulsed DC plasma source. Theenergy of the plasma source 5 is adjustable, preferably to a range of 10eV to 200 eV or also 400 eV. Preferably an ECWR(electron-cyclotron-wave-resonance) plasma source is used, in which theenergy of the plasma particles can be adjusted in the plasma source,largely independently of the plasma density.

In an area A of the vacuum chamber 10, preferably between the two areasA and B, a heating device can be arranged, preferably a radiant heaterwith quartz radiators. Alternatively, infrared radiators can also beprovided. Therewith the substrates can be heated to several hundreddegrees, to 250ÿC.

Also provided is an area C in which a second cathode sputtering source 7is placed diametrically opposite, which is preferably configured likethe first cathode sputtering source 3. In an additional embodiment,additional sputtering devices and/or plasma sources are provided in thevacuum chamber.

In the space between areas A and C an optical measuring device (opticalmonitor) 8 is arranged for optical monitoring, by means of which opticalproperties of the growing layers can be determined. Preferably,transmission and/or reflection of a coating can, as is known in itself,be measured intermittently on at least one of the substrates todetermine optical properties of the applied coating. Thus, the growingoptical layer thickness can also be checked.

A preferably planar transporter 6 moves a substrate 2 at least once pastat least one cathode sputtering source 3, 7, and past at least onestation with a plasma source 5. The transporter 6 is preferably asubstrate turntable with an adjustable speed of for example 1 to 100rpm. The acceleration to high set speeds can be performed in a fewsteps, each time within the same area A, B and C. Instead of a planartransporter a drum-shaped device, known in itself, can be used forholding and for transporting the substrates. In this case the sputteringsystem and plasma source can be associated with a peripheral surfacearea of the drum.

Usually, one or more substrates 2 are fastened on the turntable. For thesake of clarity only one of the substrates shown as a circle in FIG. 1is identified by a reference mark.

In the vacuum chamber 10 the substrate 22 is carried by the turntable 6underneath the first cathode sputtering source 3. There the target issputtered by cathode sputtering, material knocked out of the targetprecipitating onto the substrate. An argon gas-oxygen mixture is used assputtering gas in a preferred embodiment of the invention, so that thecoating growing on the substrate 2 is an oxide.

According to the invention the cathode sputtering process is operatedsuch that in the area A or C, with the input of a reactive component, acompound layer of a given composition is deposited with a predeterminedcomposition. The coating 1 is formed with at least two constituents, thereactive component O₂ forming one of the constituents and, the coating 1is produced sub-stoichiometrically with respect to component O₂. Withrespect to one of the constituents, the coating 1 is deposited with agiven deficit, of for example no more than 90, 80, 70, 60, 50, 40, 30 or20 or less atomic percent of the reactive component O₂. Then in the areaB the content of the component O₂ is increased in situ in coating 1, bymeans of plasma acting on coating 1, to the stoichiometric composition,and/or the structure of the coating is modified. Instead of the reactivecomponent O₂ another reactive gas can be fed in area B.

It is especially good if the partial pressure of the component O₂ in thearea of the cathode sputtering source 3 is adjusted during the coatingof substrate 2 to a substantially constant level. It is beneficial forthe partial pressure of the component O₂ to be regulated by controllingits flow. It is also possible constantly to adjust the oxygen partialpressure through the electrical power of the cathode sputtering source3, the rate being kept constant to an especially high degree through thelength of the target life.

Furthermore, in area A of the cathode sputtering source 3, an intensityof the plasma emission line, preferably of an emission line for thetarget material, of the reactive component or a combination of both, canbe regulated to a substantially constant level. This can be set throughthe flow of component O₂ and/or the electrical power of the cathodesputtering source 3.

The coating properties can also be varied by varying the speed at whichthe substrate 2 is carried past the plasma source 5 and/or the cathodesputtering source 3.

A coating can be produced according to the invention also through aplurality of intermediate steps by sub-stoichiometricdeposition/oxidation. Furthermore, multiple layers can be deposited inwhich alternating refraction indices are achieved along a growing totalcoating by varying the coating parameters in individual layers. Suchmultiple coatings can be deposited with control, for example, by settingthe coating and/or oxidation time and/or the number of rotations and/orby controlling the layering through the optical measuring system 8 withthe aid of the optical properties of the growing coating or sequence ofcoatings.

According to another embodiment of the invention, a coating is depositedwith a given thickness on the substrate in the area A of the cathodesputtering source 3 with the addition of a given amount of the reactivecomponent, and has less optical losses than a given minimum. In thatcase, in a known manner, the damping of a light wave falling on acoating is called optical loss. The optical losses can be learned frommeasurements of transmission and reflection. Since the stray light dueto diffuse scattering is connected with the roughness of a surface, itis possible to draw conclusions concerning the surface quality from theoptical losses. Preferably, the optical losses are determined accordingto the invention by means of the optical measuring device (opticalmonitor) 8. It is especially preferred that the optical monitor be aone-wavelength or multiple-wavelength spectrometer, especially aspectral photometer or ellipsometer, and with special preference aspectral ellipsometer. After a preset coating thickness is deposited,the optical losses are determined and there follows an adjustment ofcoating properties according to a signal from the optical monitor 8. Ifa spectral photometer is used, the transmission, absorption andreflection can be easily determined in a given spectral range and as afunction of the coating thickness.

In what follows, a description shall be given, for a preferredembodiment of the method of the invention, of the procedure forproducing a coating by reactive sputtering with a subsequentmodification of the coating applied. Other procedures are also coveredby the invention. A magnetron source system is used as the sputteringapparatus, with two magnetron systems placed side-by-side and twoniobium targets. The targets are operated alternately in a mediumfrequency range, for example with a frequency of 40 kHz. Shutters areassociated with both targets, whereby the sputtering apparatus can beisolated from the substrates. The plasma source, with which a shutter isalso associated, is operated in a radio frequency range.

In a first step the sputtering apparatus is set to a working point atwhich the shutters are closed for the stabilization of the process. Aninert gas and a reactive component (e.g., argon and oxygen) are admittedinto the range of the sputtering apparatus. Also admitted into the rangeof the sputtering apparatus are an inert gas and a reactive component.The substrate carrier, for example a planar turntable, is accelerated toa set speed. In another step the plasma of the plasma source is ignited.An ignition of the sputtering plasma of the cathode sputtering apparatusthen takes place and is brought to a specified power level. Then apartial pressure control is activated in the range of the sputteringapparatus. Preferably, a preset partial pressure level is stabilizedthrough the cathode power.

In a second step the coating of the substrates is begun. For this, theshutters are opened. It has been found that only a slight change of thecontrol parameters established at the working point by the opening ofthe shutters is required. A desired coating thickness can be controlledthrough the coating time or a number of rotations. Especially preferredis an optical measurement of the coating thickness performed in situ bythe optical monitor 8.

According to the invention, the reactive depositing of a coating isperformed at a working point on a characteristic curve or map which isselected according to the sputter material and the reactive componentmaterial to minimize the optical loss of the deposited coating or of thecoating modified by the plasma effect. A few preferred characteristiccurves are represented herewith.

In FIG. 2 a curve is presented to show the dependence of a reactive gasflow on a partial pressure of the reactive component in a cathodesputtering process, in an example using an aluminum target, with oxygenas the reactive component, and with a constant power in the sputteringapparatus (sputter power). At low levels of the partial pressure theoxygen flow first increases steeply and after a peak S it diminishes, inorder to increase again at higher partial pressure after a minimum. Atvery low oxygen partial pressure a condition establishes itself with alargely metallic target surface, at which metallic coatings aredeposited on the substrate. If the oxygen partial pressure increasesabove a level corresponding to the peak point, a transition occurs in aflow-controlled process to an oxide or compound mode, in which thetarget surface is completely coated with reaction products andstoichiometric coatings with undesirable coating qualities grow on thesubstrate. The arrow marked 0 indicates the transition to the oxide modeor compound mode. The broken curve in FIG. 2 describes the correspondingdeposition rate. It is apparent that it is maximum at a low reactive gaspartial pressure and decreases at increasing reactive gas partialpressure, until it comes into a saturation area parallel to theabscissas. Coming from the oxidic, the transition into the metallicrange first takes place at lower oxygen partial pressures, so that thecurve shows a hysteresis. The range between the apex and the minimum ofthe curve is generally inaccessible without complicated regulatingmeasures, but permits the deposition of less than stoichiometriccoatings at a high rate. The method of the invention is practicedpreferably in a given portion of the characteristic curve withincreasing or decreasing gas flow close to the apical point S, sincethere relatively high sputtering rates can be reached. Especiallypreferred is the range close to the apex S of the characteristic, with aflow of the first gas component O₂, which in the deposition of layer 1is no more than 50% below the maximum at the apex S, and with specialpreference no more than 20% to 10% below the maximum at the apex S. Inthis range a high deposition rate of a sub-stoichiometric coating isachievable, which then is exposed to plasma action. Depending on thematerial, e.g., with Ti, Nb, TiNb, it is possible according to theinvention to operate in the transition range to the right of the peak S,while for other materials, e.g., Al, Si, the range to the left of peak Sis preferred.

In FIG. 3 a curve is similarly represented for a reactive gas flow thatis held constant, at which a set value of a reactive gas partialpressure is established by means of the sputtering power. Thesub-stoichiometric range is on the left of the arrow marked 0. In thiscontrol method the sputtering is done, preferably but not exclusively,in a range around the minimum of the characteristic.

In FIG. 4 another curve is represented, in which, at constant sputteringpower, a target value of a sputtering cathode voltage is established bymeans of a reactive gas flow, and in the area on the right of thetransition marked 0 is set to a sub-stoichiometric compound. Preferredhere is an area around the peak S of the transition.

A constant reactive gas flow is used in the case of the curve in FIG. 5,a target value of a quotient of the sputtering rate and a reactive gaspartial pressure is regulated by means of the electrical power of thesputtering apparatus to a predetermined set value. The solid curve hereidentifies the characteristic line in the case of oxygen as reactivegas, while the broken curve identifies the characteristic with nitrogenas reactive gas. The arrows marked O1 and O2 indicate the transitionfrom a sub-stoichiometric regime on the right of the transition to astoichiometric regime left of the transition, for oxygen and nitrogen,respectively, as reactive gas. It can be seen that the location of thistransition depends on the reactive gas used. Also, in the case ofoxygen, the minimum is on the right of the said transition at O1, and inthe corresponding characteristic for nitrogen it vanishes; to thiscorresponds an absence of a hysteresis. The quotient of sputter rate andreactive gas partial pressure can be determined from a quotient of amaterial and reactive gas partial pressure plasma line intensity.Material in this case means the material of the sputtering cathode;silicon in the present case. A two-line measurement of this kind has theadvantage that the result is relatively independent of any contaminationof a light conductor entry window through which the correspondingemission lines are measured.

Typical values for the present invention are 40 sccm/min for the argonflow and 30 sccm/min for the oxygen flow in the area of the sputteringsystem. An oxygen partial pressure is determined preferably from thesignal from a lambda probe in the area of the sputtering system. Thetypical power of such a dual magnetron cathode station in the process ofthe invention is in the range of 20 sccm/min, while the argon flow is ina range of 2 sccm/min. The power in case of an RF operation is in arange of 1 KW.

To control the sputtering system 3, 7, and the plasma source 5 as wellas the moving of the substrates a control unit is provided which is notshown in the drawings. The control takes place in a parameter area inwhich characteristic curves and characteristic maps are plotted, asalready explained more precisely. In a preferred embodiment of theinvention a signal from the optical monitor 8 is used in order toestablish the working parameters for optimizing the optical quality,especially so as to minimize the optical losses of the depositedcoating. This is done preferably on line. Likewise, such procedure isperformed layer by layer or upon a changeover from one layer to thenext. Especially preferred is the use of an optical signal for theperformance of a control regulation to allow for long-term drifting ofthe coating properties, such as transmission, reflection, and/or opticallosses. Also, the operation of the entire apparatus comprising thesputtering system 3.7 and the plasma source 5, in regard to opticalproperties of the deposited and modified coating, or in regard to thespeed of the production of the coating. For this purpose, for example,appropriate working points are selected on a characteristic line bymeans of the control system, followed by plasma action and anoptimization value is determined.

Optical monitoring can take place directly after each sputtering by thesputtering system 3, 7, and/or after a plasma application by the plasmasource 5 on at least one substrate.

FIG. 6 shows an example of the optical transmission of a coatingproduced by the method of the invention as a function of wavelength(upper curve S₁) in comparison with a coating which was not exposedafter sub-stoichiometric deposition to the oxygen plasma from the plasmasource 5 (lower curve S₂). The coating parameters of the two layers arethe same except for the oxidation near the plasma source 5. Thesub-stoichiometric coating shows a very low transmission, but very highlosses, so that it is unusable as an antireflection coating or filter orthe like. It can be seen plainly that oxidation by the plasma actionpermits a very effective improvement of the coating properties (uppercurve S₁).

FIG. 7 shows an example of the optical transmission of a coating as afunction of wavelength, with a substrate speed as a parameter of thecurves. At a high speed of, e.g., 180 or 120 rpm, the opticaltransmission of the coating is higher (upper curve) than with only halfthe speed of, e.g., 60 rpm (lower curve).

FIG. 8 shows examples of single-layer coatings of various thicknesswhich were made according to the invention. Curves A and B identifyNb₂O₅ coatings with a thickness of 1,000 nm to 500 nm. Curve Cidentifies a Ta₂O₅ single layer with a thickness of 1,000 nm. Curves Dand E identify SiO₂ coatings with thicknesses of 1,000 nm and 500 nm,respectively. It can be seen that the optical losses of the materialused depend on the coating thickness and the wavelength. All in all theoptical losses are very slight and increase only near the absorptionedge of the material in question.

Single layers of a high-refracting material, such as Nb₂O₃, Ta₂O₅, TiO₂,ZrO₂, Al₂O₃ require for a low optical loss a deposition in the reactivesputtering with only a slight oxygen deposit, the coatings beingthereafter exposed to the reactive plasma of the plasma source. Theenergy of the particles of the reactive plasma of the plasma source 5 ispreferably lower than 50 eV. For low-refraction single layers such asSiO₂ can also be obtained with a greater oxygen deficit in the reactivesputtering followed by the action of the reactive plasma of the plasmasource.

According to the invention, an excellent optical quality of the coatingsresults if they are first made sub-stoichiometrically with a definedoxygen deficiency and are then oxidized by plasma action to thestoichiometric oxide. Typically, 0.2 to 0.4 nm are deposited perrevolution. The deposited coating is preferably X-ray amorphous ornanocrystalline with a smooth surface, but at the same time has a densetexture free of voids, so that the prevention of water entry from theatmosphere is achieved, which otherwise would lead to unwantedrefraction changes. The improved surface texture is to be attributedsubstantially to the plasma action which to this extent can replace anytreatment of the substrate with a bias voltage common in the state ofthe art.

In the production of low-loss multilayer coatings according to theinvention, in which a deposit of a second layer containing at least onereactive component on a first layer is performed by means of areactively operated coating apparatus, an alteration of the textureand/or stoichiometry of the coatings is likewise provided by means of areaction system. The first layer can also be substrate. A schematicrepresentation of a multi-layered coating on a substrate, with a firstlayer, a second layer and an interface of the thickness d in an area ofthe second layer adjoining the first layer is shown in FIG. 9. Thecoating apparatus can be any reactively operated coating apparatus,especially apparatus which operate on the principle of physical vapordeposition, such as vapor depositing or sputter technologies. Thereaction apparatus is a plasma source, e.g., a DC, HF, MF or DC pulse orDC plus HF or microwave plasma apparatus.

Preferred are multi-layer coatings of alternating high- andlow-refraction layers, such as are used especially for optical filters.The multilayer coatings can also consist of alternating high-, low- andmedium-refracting layers. A layer with an index of refractionsubstantially greater than 1.9, preferably between 1.9 and 2.6, isconsidered as a high-refraction layer.

A coating with a refractive index between 1.3 and 1.5 is considered tobe low-refraction. Medium-refraction coatings have a refraction indexExamples of high-refraction materials in this sense are, for example,Nb₂O₅, Ta₂O₅, or ZrO₂. Coatings of SiO₂, for example, are consideredlow-refraction.

In FIG. 10 are shown transmission and reflection values with a number ofN₁=21 and N₁=79 interfaces for Nb₂O₂/SiO₂ multilayer coatings inrelation to wavelength. From the figure it can be seen that with agrowing number of interfaces or of layers of the multilayer coating theoptical losses increase.

The depositing of the coatings as well as the alteration of the textureand/or stoichiometry of the layers is performed preferably by the methoddescribed further above, yet other methods can also be practiced. Tolower the optical loss of the multilayer coating below a given level, aninterface with a thickness d1 is created in an area of the second layeradjoining the first layer, preferably by means of the coating apparatus,and with a value of a deficit DEF of the reactive component lower than avalue DEF₁.

It is preferred in the method of the invention if the thickness d₁ ofthe interface exceeds a minimum value. The rest of the deposited layercan be deposited reactively with a higher deficit DEF.

With special preference the interface is produced by operating thereaction apparatus with a stoichiometry as perfect as possible.

Preferentially in the described process a low-refraction coating of Si02is deposited on a high-refraction coating, for example, Nb₂O₅, Ta₂O₅ orthe like. Carbon or oxygen also can be used as the reactive component. Ahigh-refraction layer can again be deposited on the low-refractionlayer.

A multilayer coating according to the invention can be produced indifferent ways. It is especially simple if values of a momentarythickness d(t) of the second layer are determined and, as soon as d(t)is greater than a value d₁, the deposition of the second layer is begunwith a value of the reactive component deficit DEF is greater than DEF₁.The values of the momentary thickness d(t) of the second layer can bedetermined during the deposition of the second layer, for exampleaccording to a monitoring signal from the optical monitoring system (8).

In FIG. 11 is shown the effect of minimizing the optical loss for amultilayer coating made by the method of the invention. In a Nb₂O₅/SiO₂multilayer coating, the interfaces have been deposited with differentoxygen deficits. A and Aÿ represent the reflection and transmissionvalues in relation to wavelength for a multilayer coating with arelatively great oxygen deficit at the interface, while B and Bÿ are thetransmission and reflection values with a relatively low oxygen deficitat interface. Curves A and Aÿ show greater optical losses than curves Band Bÿ.

In FIG. 12, to further illustrate the method of the invention for singlelayers of high- and low-refraction material and for multiple layercoatings with alternating optical layers of high and low refraction,optical losses are plotted in FIG. 12 in relation to wavelength. Herethe values for a multilayer coating with 77 non-optimized interfaces areindicated at A, while values of an otherwise equal multilayer coatingare given at B. The losses of B are lower than those of A, because thenumber of interfaces is only 21 compared with 77. The values of amultilayer coating comparable to A, however, but with optimizedinterfaces, are indicated by XY. The curve XY shows extremely lowerlosses compared with curve A for the corresponding multilayer coating.Curves C, D, e and F designate high- and low-refraction single layerswith a thickness of 1,000 nm and 500 nm, respectively.

FIG. 13 shows transmission and reflection in relation to wavelength fora N₂O₅/SiO₂ multilayer coating which is made by the method of theinvention to achieve the lowest optical losses. If the transmission andreflection of a filter is measured in relation to wavelength, the filterhas places of maximum and minimum transmission. At the maximumtransmission points the reflection is minimal and vice versa. At thesepoints the losses can most easily be determined: by subtracting from100% Tmax and Rmin or Tmin ant Rmax. A multilayer coating according toFIG. 13 is designed preferentially for broadband filters.

FIG. 14 illustrates, for a SiO₂ interface layer on a high-refractioncoating, the effect of a variation of the thickness d of the interfacelayer on the optical losses in relation to wavelength. The curve markedA gives values for a 1.7 mm thick interface layer which shows a clearrelationship to wavelength. In comparison, the values for a 3.6 mm thickinterface layer is marked B and shows considerably lower optical lossesas well as a considerable independence from the wavelength in the rangerepresented. In FIG. 14 it is furthermore apparent that a criticalthickness of the interface layer exists, after which a clear reductionof the optical losses occurs if the value of the deficit of reactivecomponent DEF is made less than a critical value DEF₁. In the case of alow-refraction layer SO₂ on a high-refraction layer, e.g., Nb₂O₅, thecritical value is in a range between 2.5 and 10.0 nm, preferably 2.6 nm,27 nm . . . . 3.6 nm, 3.7 nm, 3.8 nm. The deficit DEF=2−x of thereactive component is then to be chosen relatively low, corresponding toa value x of the reactive component of SO_(x) in the area of theinterfaces of more than 1.5, 1.6 . . . to 1.8.

In a preferred embodiment of the method, in order to produce the SiOxcoating in the interface area, at first relatively thin layers areproduced per rotation of the substrate plate at relatively lowsputtering power and a relatively low oxygen deficit. Preferablyreactive sputtering is performed in the area of transition to the oxidemode. After reaching a coating thickness, which can be preset, of forexample 3.6 nm, the SiOx coating is deposited with a relatively greatoxygen deficit at a higher power and exposed to the reactive plasma fromthe plasma source. This [plasma] is preferably constant.

In FIG. 15, curves are shown for the reactive deposition of theinterface coating at a lower and greater deficit. The figures for thesputtering voltage and initial power of a lambda sensor at 1 kW areindicated at A and Aÿ, and figures for the sputtering voltage and thelambda signal at a sputtering power of 1.5 kW are given at B and Bÿ. Theenergy of the coating particles is preferably between a few eV up to 200eV. It is preferred if the particles of the reactive plasma have anenergy of less than 50 eV.

In FIG. 16, the time curves of important process parameters duringreactive cathode sputtering with oxygen as a reactive component arerepresented schematically, as used in a preferred embodiment of theprocess. In this case the operation is performed at constant values ofthe plasma source power, of the argon flow in the area of the sputteringapparatus and of the oxygen flow in the rea of the sputtering source andplasma source. At a moment T_(c) the power of the sputtering system isincreased. From this moment T_(c) the oxygen partial pressure in rangeof the sputtering apparatus is reduced. Preferably, at the moment T_(c)a specified coating thickness of, e.g., 3 nm is reached. The coatingsputtered up to this moment has a relatively low oxygen deficit. Layerssputtered after the moment T_(c) have a relatively high oxygen deficit.It is understood that, in further embodiments of the invention, theparameter values of other factors than those of the sputtering power andoxygen partial pressure can vary differently time-wise in the area ofthe sputtering apparatus.

By the deposition of relatively thin SiOx coatings with a relatively lowoxygen deficit it can be assured that both the interface layer and thelayer, for example of Nb₂O₅, underneath it, by dint of the action of thereactive plasma source following the SiOx deposition, can be astoichiometric coating. Especially it can be achieved by such aprocedure that any eventual withdrawal of the reactive component,especially the oxygen, can be compensated by the reactive SiOx from thelayer beneath it (Nb₂O₅) due to the action of the plasma source. If onthe other hand the SiOx is deposited at too high a rate, it is possibleby the reactive plasma action to oxidize to a stoichiometric SiO₂, butit becomes under certain circumstances so dense that a sufficientpenetration of the action of the plasma source to achieve stoichiometryof the layer underneath it to the layer under the SOx layer is notpossible. After a certain thickness of the interface layer applied witha slight oxygen deficit, SiOx applied anew onto the already depositedlayer can withdraw no more oxygen from the layer underneath it, so thatthe further deposition of SiOx with a higher deficit of oxygen can takeplace.

1. An apparatus for producing a coating on a substrate with a sputteringapparatus in a vacuum chamber with a residual gas, said coating beingformed from at least two constituents and at least a first constituentis a sputter material of said sputtering apparatus and at least a secondconstituent is a reactive component of said residual gas, comprising: adevice for reactive depositing said coating on said substrate whilefeeding a reactive component with a predetermined stoichiometric deficitin the vicinity of said sputtering apparatus; a device for moving saidsubstrate with said deposited coating in the vicinity of a plasma sourcewhich is arranged in said vacuum chamber at a predetermined distancefrom said sputtering apparatus; and a device for modifying textureand/or stoichiometry of said coating via a plasma treatment by saidplasma source while supplying a predetermined amount of the reactivecomponent to reduce an optical loss in said coating.
 2. An apparatus forproducing a coating on a substrate by with a sputtering apparatus in avacuum chamber with a residual gas, said coating being formed of atleast two components, and at least a first constituent is a sputtermaterial from said sputtering apparatus and at least a secondconstituent is a reactive component of said residual gas, comprising: adevice for reactive depositing said coating on said substrate in thevicinity of said sputtering apparatus while supplying a reactivecomponent with an optical loss falling below a minimum at a givencoating thickness; a device for moving said substrate with saiddeposited coating in the vicinity of a plasma source which is arrangedin said vacuum chamber at a predetermined distance from said sputteringapparatus; and a device for modifying structure and/or stoichiometry ofsaid coating via a plasma treatment by said plasma source whilesupplying a predetermined amount of the reactive component to reduce anoptical loss in said coating.
 3. The apparatus of claim 2, furthercomprising a gas supply and/or a pump unit arranged in the vicinity ofsaid sputtering apparatus and said plasma source and having apass-through for at least one substrate.
 4. The apparatus of claim 2,wherein said substrate is arranged on a turntable spaced away from saidsputtering apparatus and said plasma source.
 5. The apparatus of claim4, wherein several substrates are arranged on said turntable.
 6. Theapparatus of claim 4, wherein said sputtering apparatus and said plasmasource are arranged to correspond to the circumference of saidturntable.
 7. The apparatus of claim 2, further comprising at least twosputtering apparatus disposed diametrically opposite one another.
 8. Theapparatus of claim 7, wherein said plasma source is arranged spatiallybetween said two sputtering apparatus.
 9. The apparatus of claim 7,further comprising an optical measuring device, arranged spatiallybetween said two sputtering apparatus, for measuring an opticaltransmission, reflection and/or loss of said coating deposited on saidsubstrate.
 10. The apparatus of claim 7, further comprising at least oneoptical measuring apparatus disposed spatially between said twosputtering apparatus.
 11. The apparatus of claim 9, wherein said opticalmeasuring apparatus is an one-wavelength or multiple wavelengthphotometer and/or ellipsometer.
 12. The apparatus of claim 2, furthercomprising a heating system, arranged at least in an area of said vacuumchamber, for heating said substrate.
 13. The apparatus of claim 2,wherein said sputtering apparatus is a magnetron-supported cathodesputtering source.
 14. The apparatus of claim 2, wherein said sputteringapparatus is operated with an alternating electric field in ahigh-frequency, medium-frequency or pulsed DC range.
 15. The apparatusof claim 2, wherein said plasma source is one of the following: anelectron-cyclotron-wave-resonance (ECWR) source, a Hall End plasmasource, a hot cathode DC plasma source, a high-frequency plasma source,a medium-frequency or pulsed DC plasma source.
 16. An apparatus forproducing a multilayer coating, comprising: at least one reactivelyoperated coating apparatus; at least one reaction apparatus in a vacuumchamber; and a control system; wherein said coating apparatus isoperable to deposit a second layer with at least one reactive componenton at least one substrate moving relative to said coating apparatus andsaid reaction apparatus; wherein said reaction apparatus is operable tochange texture and/or stoichiometry of at least one layer; and whereinsaid control system is operable to control said coating apparatus andsaid reaction apparatus to form an interface layer, in a region of saidsecond layer adjoining a first layer, with a thickness d₁ and a value ofa deficit of said reactive component DEF lower than a value DEF₁ toreduce an optical loss in said multilayer coating below a predeterminedvalue.